In order to prevent an accident, it is important that a charging voltage does not exceed a rated voltage and a charging current does not exceed the maximum allowed current while charging a lithium ion secondary battery. It is necessary to manage both the charging voltage and the charging current, and constant-current constant-voltage charging system is employed. Commercially available charging apparatuses are configured to enable strict management of the charging voltage and the charging current. However, it is difficult to say that a sufficient safety measure is provided, only with strict management of the charging voltage and the charging voltage provides. Therefore, it is under consideration to detect a temperature within a battery pack and to perform charging only when the temperature is within a safe range.
For example, a temperature range of 0° C. to 45° C. is set as a chargeable temperature range (hereinafter referred to as a “charge temperature range”). When a lithium ion secondary battery is charged at a low temperature, lithium ions emitted from the positive electrode become poorly absorbed by the negative electrode, and a lithium metal become easily deposited. This might cause such an accident that the deposited lithium metal breaks through a separator. In addition, when charged at a high temperature, temperature rise due to the charging current is added to an environmental temperature, and the temperature of the battery rises. This might make it difficult to maintain the function of the separator, leading to an accident.
In related art charging apparatuses for charging a nickel hydrogen battery or a nickel cadmium battery at a constant current, charge completion time detection (so-called ΔT detection mode) is carried out by causing a temperature detecting element to detect a rapid increase in battery surface temperature per a certain period of time. For example, Japanese Unexamined Patent Application Publication No. Hei 7-15885 describes that a reference voltage value at a full charge is corrected depending on a temperature change rate, in order to solve a problem of incorrect charging under the influence of temperature in the ΔT detection mode.
The present application disclosed in the above publication is not intended to solve a problem of a case where the lithium ion secondary battery is charged beyond the charge temperature range. Unless attentions are paid to the heat generated by the battery and to the heat generated from the circuits of the charging apparatus, the battery temperature cannot be detected with high accuracy.
It is desirable to provide a charging apparatus and a charging method that can surely perform charging within the charge temperature range.